Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus which has an illumination system and a projection objective. The image of a mask (=reticle), illuminated via the illumination system, is projected onto a substrate via the projection objective. The substrate (for example a silicon wafer) is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure onto the light-sensitive coating on the substrate.
In projection exposure apparatuses designed for the EUV range (wavelengths of for example about 13 nm or about 7 nm) mirrors are used as optical components for the imaging process due to the general lack of availability of suitable translucent refractive materials. In practice, operation under EUV conditions involves implementing many functionalities and fulfilling demanding desired properties.
The service life of the mirrors or the projection exposure apparatus designed for operation under EUV conditions is usually limited due to contaminating particles or gases, in particular hydrocarbon compounds, so that operation of the projection exposure apparatus is generally performed under vacuum conditions (for example at total pressures of 10−3 mbars or below). The contaminants which spread in the system can adhere to the surfaces of the optical elements, which in turn results in an adverse effect on the optical properties of the elements such as for example a loss of reflection in respect of the mirrors.
WO 2008/034582 A2 discloses among other things an optical arrangement, in particular a projection exposure apparatus for EUV lithography, which, to reduce the adhesion of contaminants, in particular to reflective optical elements, has within an evacuated housing at least one further vacuum housing surrounding the optical surface of the respective reflective optical element. Associated with the vacuum housing is a contamination reduction unit which reduces the partial pressure of contaminating substances such as water and/or hydrocarbons at least in the immediate proximity of the optical surface. In that way a kind of “mini-environment” is generated around the optical surface, with a reduced number of contaminating particles, so that fewer particles can be deposited on the optical surface.
US 2009/0135386 A1 discloses among other things the provision of a plurality of subchambers in an illumination system of a projection exposure apparatus within a vacuum chamber. The subchambers are separated from each other by way of separating walls provided with a passage opening therethrough and respectively arranged at positions of minimum light cross-sectional area or in the proximity thereof, and respectively evacuated by associated vacuum pumps.
In addition, in operation with globally or locally high levels of light power density, the increase in temperature of the optical elements such as for example mirrors, lenses or holder elements, that is involved with the high light power density and absorption, can result in an adverse effect on the imaging properties of the optical system. An example of this is the adverse effect of temperature-sensitive elements present in the optical system such as for example position sensors. It is known for example in projection objectives designed for the EUV range, in addition to a carrier structure which carries mirrors and mirror actuators, to provide a measurement structure which is typically arranged outside the carrier structure and which is intended to ensure thermally and mechanically stable fixing of position sensors or other measurement systems for determining the mirror positions. An unwanted rise in temperature of that measurement structure is correspondingly more serious as the spacing between the position sensors and the mirrors which rise in temperature in operation of the projection exposure apparatus is relatively small (and for example can be in the range of between 5 and 100 mm).
It is known from US 2005/0018154 A1 to provide in a microlithographic projection exposure apparatus at least one heat shield which is intended to absorb the heat given off by the mirrors and/or the carrier structure thereof, wherein that heat is dissipated by a heat transport circuit which is in mechanical contact with the heat shield.